Common Problems and Solutions of Injection Molding - Part 1

Introduction of Injection Molding and Its Problems

Injection molding is a molding method including injection and molding. Its  advantages are: 

  • Fastproduction speed
  • High efficiency
  • Automatic operation
  • Various colors, shapes and sizes
  • Accurate size
  • Easy to replace

Injection molding is suitable for mass production and complex shape products and other molding processing fields.

In the daily injection molding process, we often encounter various quality problems. The quality problems of injection molding products include minor surface defects and more serious quality problems. 

And they will affect the performance and safety of the products. And they are usually caused by problems with the molding process, material usage, mold design, or a combination of the three. Optimal process settings are critical to final productivity levels, part quality and final injection molding cost. Today, we will systematically analyze the common injection molding problems and solutions. And this topic is divided into 2 Parts.

Defects of Injection Molding

Problem 1: Short Shots

Short Shots

Short shots occur when the flow of molten material does not completely fill the cavities in the mold. Then, it results in an incomplete molded part after cooling. Usually, this will cause the thin walls (or combinations), or molded parts filled with polymers to be far away from the gate, especially for the long flow distances, resulting in functional or appearance defects in the product.

Common Causes

  • Insufficient amount of material injected
  • Insufficient injection filling speed
  • Insufficient pressure
  • Insufficient injection time
  • Unbalanced multi-cavity molds
  • A foreign object is blocking the nozzle
  • Overly low injection temperature
  • Overly small gate or vent
  • Overly low mold temperature
  • Excessive wear on the screw& barrel
Short shots-1


  • Redesign the mold with wider channels or gates for better flow.
  • Increase injection speed or pressure to improve flow.
  • Choose thinner substrates to improve flow.
  • Increase mold temperature to prevent material from cooling too quickly.
  • Add additional vents or expand existing vents in the mold to allow trapped air to escape.

Problem 2: Jet

Jetting refers to a deformation of the molded part. When an initial “jet” of molten material is injected into a mold cavity, the air within that cavity begins to solidify before the cavity fills.

Jets typically appear as wavy lines on the surface of the finished part, usually starting from the initial gate of the injection. This visible flow pattern could lead to some weakness.


Common Causes

When molten polymer or other material is injected under high pressure through a small gate, it usually spews out of the gate quickly rather than gradually filling the cavity.

As the first line of this material cools the mold walls and begins to harden, the remaining mold material pushes it, leaving an impression on the surface of the finished part.


  • Reduce injection pressure to prevent rapid injection of material into the cavity.
  • Increase material and mold temperature to prevent early solidification of initial material jet.
  • Design the mold with an injection gate so that the material goes through the mold rather than longitudinally.

Problem 3: Flash

Flash (also known as “splatter” or “burr”) is excess molding material that appears as a thin lip or protrusion at the edge of the component. Flashing occurs because material has flowed out of the intended runner and into the space between the tool plates or at the injector pins.

Flash usually appears near seams, sealing surfaces, exhaust passages or injectors.


Common Causes

  • Overly high injection pressure(filling or packing)
  • Foreign objects on the mold surface (contamination, dirt, etc.)
  • Overly high injection and mold temperature
  • Resin melt viscosity is too low
  • The surface of the closing mold does not match


  • If the plates do not fit together properly or allow material to flow out of the channel, the mold can be redesigned.
  • Increased plate holding force to restrict material flow to channels.
  • Adjust mold temperature, injection pressure and ventilation to improve material flow.

Problem 4: Sink marks

Sink marks are an aesthetic flaw in the surface. They appear as depressions, usually in thicker parts of the molding. The root cause is usually shrinkage of the material during crystallization.

Sink marks

Sink marks are mainly caused by thermal shrinkage (shrinkage) during cooling. After the outer material cools and solidifies, the inner material begins to cool. Its shrinkage pulls the outer material (surface) inward, causing sink marks.

Common Causes

  • Insufficient injection pressure
  • Insufficient dwell time
  • Overly high injection speed
  • Insufficient amount of material
  • Insufficient cooling or injection time
  • Melt or mold temperature is too high
  • Poor part design, non-uniform walls and/or excessive wall thickness


  • Increase hold pressure and time to cool material near part surface
  • Design molds with thinner component walls for faster cooling near the surface
  • Increased cooldown to limit shrinkage

Problem 5: Voids (bubbles)

Voids appear in moldings as small air bubbles or bubbles, usually created by air, gas, or vacuum. If the item is highly transparent, the voids become more noticeable. One of the main reasons for voids is that the molding pressure is not enough to force entrapped air out of the mold cavity.

Voids (bubbles)

Common Causes

  • Incomplete filling of the mold
  • Poor mold venting, especially around protrusions
  • Fill rate is too fast (trapped air produces short shots)
  • Mismatchmold temperature
  • Parts are too thick
  • Wrapped in moisture
  • Porous or very fine additive powder that absorbs air to encapsulate the air


  • Increase injection pressure to force trapped air bubbles.
  • Choose a lower viscosity material to limit the risk of bubble formation.
  • Place the gate near the thickest part of the mold to prevent the material from being susceptible to premature cooling by voids.

Problem 6: Warpage

Warpage is a deformation that can occur in an injection molded product when different parts of the part shrink unevenly. Just as wood warps when it dries unevenly, plastics and other materials warp during cooling.

Common Causes

Because uneven shrinkage can put unnecessary stress on different areas of the molded part. This undue pressure can cause the finished part to bend or twist as it cools. One of the main reasons for warping of injection molded plastics and similar materials is that cooling occurs too quickly. Excessive temperature or low thermal conductivity of the molten material can exacerbate the problem.



  • Make sure the cooling process is gradual and long enough to prevent uneven stress on the material.
  • Reduce material or mold temperature.
  • Try switching to a material that shrinks less during cooling (eg, pellet-filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades).
  • Redesigned mold with uniform wall thickness and partial symmetry to ensure greater part stability during cooling.

Problem 7: Burn marks

Burnt marks are defects caused by polymer degradation during injection molding, resulting in black or rust-colored discolorations on the edges or surfaces of molded plastic parts.

Voids (bubbles)

Common Causes

  • Injection pressure is too high
  • Injection speedis too high
  • Back pressureis too high
  • Temperature is too high
  • Insufficient exhaust
  • Improper runner location or design
  • Other material contamination


  • Lower melt and mold temperatures to prevent overheating.
  • Reduced injection speed to limit the risk of air trapping in the mold.
  • Enlarge gas vents and gates to allow trapped air to escape the mold.
  • Shorten mold cycle times,sothat any air and resin don’t overheat.

Problem 8: Discoloration

Discoloration or “staining” occurs when a molded part has a different color than expected. Often the discoloration is confined to a localized area or a few unusual lines of color on the molded part. This defect usually affects the appearance of the part without reducing its strength.

Common Causes

  • Leftover pellets in the hopper or residual resin in the nozzle or mold from a previous production process
  • Poor thermal stability of the colorant or improper mixing of the masterbatch


  • Ensure workers properly clean hoppers, nozzles and molds between production runs to remove any residual particles or substrate.
  • Consider using a cleaning solution to remove excess color from the machine.
  • Make sure you or your supplier use colorants with adequate thermal stability.
  • Ensure that the masterbatch is evenly mixed for consistent color output.

Problem 9: Weld lines

Weld lines represent the optical and mechanical defects in molded parts. Weld lines typically appear in the areas where the polymers flow together during the injection molding process. Flow marks appear due to flow turbulence during injection.

Weld lines may appear on the surface of the molded part where the molten material converges after it has split into two or more directions in the mold. Hair-like weld lines are the result of weak material adhesion, which reduces the strength of the part.

Weld lines

Common Causes

  • Injection temperatureis too low
  • Mold temperature is uneven or too low
  • Mold filling is too fast or too slow
  • Too much mold release agent or lubricant
  • The mold surface is scratched or dirty
  • Fill speed and/or time is too low
  • Insufficient exhaust


  • Increase material temperature to prevent local solidification.
  • Increase injection speed and pressure to limit cooling before material fills the mold.
  • Redesignmold to eliminate partitions.
  • Switch to a lower melting point or lower viscosity material for faster flow and prevent premature cooling.

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